Numerical study of using dual sources constructed via deconvolution to suppress the collar waves in acoustic logging while drilling

Abstract

In the design of acoustic logging while drilling (LWD) instruments, acoustic isolation is crucial to prevent the loss of formation signals due to collar waves. A conventional approach involves carving periodic grooves on the drill collar to isolate collar waves, but this can compromise the mechanical integrity of the collar. In this paper, we introduce a novel method using dual sources to suppress collar waves generated by a monopole source. This method added a new near-source to the original acoustic LWD instrument, and its input signal is constructed via deconvolution. The effectiveness of this method is first verified using the finite-difference time domain (FDTD) in an infinite fluid medium. The amplitude ratio of the collar wave’s average frequency spectrum (0–20 kHz), excited by the deconvolution-type dual sources (DCDS), compared to a single far-source, is only 1.57%, effectively suppressing most of the collar waves. Since formations do not significantly affect the propagation characteristics of collar waves, this method remains effective in various formations, as corroborated by simulation results. Additionally, we applied the wave interference-based slowness-time-coherence (STC) method to successfully extract the formation P-wave velocity even when collar wave velocity and formation P-wave velocity are similar. This method provides an effective approach to suppress monopole collar waves without compromising the collar’s integrity and mechanical strength.